Properties of Fuel Oil – Explanation of Key Terms

(a) Calculated Carbon Aromaticity Index (CCAI)

The Calculated Carbon Aromaticity Index (CCAI) is a numerical value used to indicate the ignition quality of residual fuels such as Heavy Fuel Oil (HFO). Unlike distillate fuels, which use the Cetane Index, HFO requires CCAI because its ignition characteristics depend mainly on its density and viscosity.

Calculation:

CCAI is determined using:

• Fuel density at 15°C
• Kinematic viscosity

Effect on Engine Performance:

• High CCAI (e.g., > 860):
  • Indicates poor ignition quality (long ignition delay)
  • Causes sudden pressure rise during combustion (engine knocking)
  • Leads to high mechanical stresses on bearings
  • May result in damage to piston rings
• Low CCAI:
  • Indicates better ignition quality
  • Fuel ignites more readily after injection
  • Ensures smoother and more efficient combustion

  (b) Open Flash Point and Closed Flash Point

  Flash point is the lowest temperature at which a fuel produces enough vapour to form a flammable mixture with air.

  Types of Flash Point:

  • Closed Flash Point (Pensky-Martens Apparatus):
    • Measured in a closed container
    • Vapours are confined, so ignition occurs at a lower temperature
    • Used as the standard for maritime safety regulations (SOLAS)
    • Minimum required flash point for engine room fuel oil is generally 60°C
  • Open Flash Point (Cleveland Open Cup):
    • Measured in an open container
    • Vapours can escape, so ignition occurs at a higher temperature than in closed conditions

    Safety Importance:

    • Fuel temperature in settling and service tanks must be maintained below the flash point (unless specially designed systems are used)
    • Prevents risk of fire and explosion in the engine room

    (c) Importance of Sodium to Vanadium Ratio

    The Sodium (Na) to Vanadium (V) ratio is a key factor in determining the risk of high-temperature corrosion in engine components such as:

    • Exhaust valves
    • Turbocharger turbine blades

    Chemical Behaviour:

    • Sodium and Vanadium are naturally present impurities in HFO
    • During combustion, they react to form sodium vanadyl vanadates

    Critical Issue (Low Melting Point):

    • These compounds melt at temperatures as low as ~530°C
    • Form sticky molten ash that adheres to hot metal surfaces

    Consequences:

    • Molten ash acts as a flux, dissolving the protective oxide layer on metal surfaces
    • Leads to:
      • “Wire drawing” of exhaust valves
      • Rapid corrosion and burnout

      Recommended Ratio (Golden Rule):

      • Sodium to Vanadium ratio should be below 1:3
      • Increased sodium (often due to seawater contamination) lowers ash melting point further, accelerating corrosion

      (d) Octane Number

      The Octane Number measures a fuel’s resistance to knocking (pre-ignition) in spark-ignition (SI) engines, such as petrol engines.

      Working Principle:

      • A higher Octane Number means the fuel can withstand higher compression before auto-ignition
      • This ensures smooth combustion without knocking

      Marine Relevance:

      Although not used in diesel engines (which rely on Cetane Number), Octane rating is important in:

      • Gasoline-operated lifeboats and rescue boats
      • Dual-fuel engines operating in gas mode

      Equivalent Concept:

      • In gas engines (e.g., LNG systems), the Methane Number is used
      • It is similar to Octane Number and indicates resistance to knocking in gaseous fuels